Mixed liquids having differing specific gravities will naturally stratify while standing in a container. This occurs with many liquids in many different disciplines. For example, liquids from gas wells that are separated from the gas and stored in a tank, such as a 400-barrel or 500-barrel tank, stratify. These liquids include hydrocarbons, water and various contaminants. The lightest (lowest specific gravity) liquid is clean oil and condensate and forms as the top layer (oil floats on water). The next layers from top down are dirty oil (a layer of dirty oil, contaminates and water) waste oil, water, and a bottom layer of sediment and water.
The valuable clean oil is presently accessed for removal and sale by first removing the lower levels of liquid through a fixed position outlet near the bottom of the tank. This is a time consuming and expensive process and involves unnecessary vehicular traffic.
The invention will be described in connection with stratified liquids from gas and oil wells. However, the apparatus and methods of liquid management, both manually and by automation, are not limited to these stratified liquids. The apparatus is designed for and is most useful in managing stratified liquids in a closed container where the liquids include potentially explosive gases, corrosive material, and/or poisonous material. The various methods of managing liquids are useful in accessing and removing any selected layer of stratified liquids. Additionally, the apparatus and other methods of managing liquids are useful in adding a material in some form to one or more of the stratified liquids. The material added may be an emulsifier or flocculent or some other material that may aid separation and stratification without mixing or contaminating neighboring layers of liquid, or may serve some other purpose.
In discussing this invention in the context of gas wells, the following definitions are applicable:
“Oil” means all liquid hydrocarbons and particularly that produced from or for the benefit of jurisdictional leases, including condensate and oil from tar sands that is measured as a liquid. See the Onshore Oil and Gas Order No. 4; Measurement of Oil brochure issued by the Bureau of Land Management (“BLM”) under 43 CFR 3160, published in the Federal Register/Volume 54, No. 36, Feb. 24, 1989 and effective Aug. 23, 1989.
“Clean Oil” means crude oil or condensate that is clear and can be sold “as is” to refineries or third parties. Some refineries require that there be less than 1% impurities, while others will accept up to 1½% impurities.
“Dirty Oil” also called “Slop Oil” means oil that has economic value but needs more processing; such as application of heat, emulsion breakers, and time to settle out suspended sediments or undesirable liquids. Dirty oil has more impurities than acceptable to the refinery receiving the oil.
“Waste Oil” is lease crude oil that has been determined by the authorized officer to be of such quality that it cannot be treated economically and put in a marketable condition with existing or modified lease facilities or portable equipment and cannot be sold to reclaimers and also has been determined by the authorized officer to have no economic value and for which royalty is not due. See the BLM brochure Onshore Oil and Gas Order No. 4, supra. The liquids and contaminates in this zone have a specific gravity less than water but greater than oil. The liquids in this zone have little value (depending on recoverable hydrocarbons), and are expensive to dispose of properly. In the oil and gas industry this zone of waste oil is often referred to as the interface layer. For purposes of clarity and a more general description the term “interface will mean, in this application, the surface at which two differing fluids, particularly liquids, meet.
“Produced Water” means the water that has been extracted from a well along with hydrocarbons and contaminates. It is mostly water but contains dissolved solids such as chlorides, sulfides, sodium, calcium, carbonates, some gases and oils, along with bacteria. Produced water is governed by regulating agencies and must meet specific criteria to be reused or disposed.
“Pumper” is the operator who visits various oil and gas well locations. He is responsible for managing the well in a profitable and appropriate manner. His job includes scheduling water trucks, oil transports, and maintenance of all equipment.
“Water truck” is a truck which has a tank mounted to it that fills by creating a vacuum for loading, and is unloaded by generating pressure to push water out. Typical water trucks are bobtail trucks and transport trucks. Most bobtail water trucks haul 80-90 barrels (bbls). The larger transport type water trucks haul 120-150 bbls.
“Clean Oil Transport” is a truck with a semi-type tank trailer attached to it. This truck is also called an “Oil Truck” and is specially equipped to haul oil products in a safe manner. This type of truck is required to move oil from well sites, compressor stations and water plants to a refinery or to another site. Generally these trucks haul about 200 bbls of clean oil. Some of these trucks are equipped with a centrifuge to test an oil sample before loading. Alternatively, a pumper or someone responsible for tracking the transfer of oil, will monitor a LACT unit or some other means of measuring the amount of oil and the purity of the oil transferred. If the sample test has a greater percentage of impurities than acceptable, the load is rejected and is not loaded. If the contaminated oil is loaded, it will generally be rejected at the refinery or by the intended party.
“Hot Oil Truck” is a specially designed truck-mounted heating unit which circulates liquids from a tank through a heat exchanger and back into the tank. This process cleans up the oil by aiding separation of oil from the water and causing the suspended sediments to drop to the tank bottom and the clean oil to rise to the top.
“Sludge” means bottom or base sediments and water or BS&W. It has no value but still must be disposed of, which is expensive.
“Heater/Treater Separator” is a vessel which uses heat to separate oil from water. Water is discharged from the bottom of the unit and oil from the upper portion.
“Settling Tank” is a container where water processed through the Heater/Treater Separator is held prior to further processing or disposal, as regulated. Some BS&W settles to the bottom of this tank and is disposed of. Valued hydrocarbons rise to the upper liquid range and should be recovered. Hydrocarbon recovery prevents contaminating disposal wells, evaporation ponds, or reverse osmosis units which receive the water from the tanks.
A plurality of gas wells are typically drilled from one pad. The wells are drilled in different directions and are often terminated under land owned by different people. Consequently, the quantity of oil, which is one of the liquid byproducts of a gas well, must be tracked for each well for proper payment of royalties. The liquid byproducts of each separately owned well are stored in individual storage tanks. There are often many storage tanks on a well pad where there are a plurality of wells and owners. It is not uncommon in some gas fields for there to be six or more 500-barrel storage tanks on a single pad.
The liquids in these storage tanks stratify because of the different specific gravities. The heavier water goes to the bottom of the tank and the lighter oil goes to the top of the liquids in the tank. Over time, BS&W (heavier material such as dirt and sand mixed with water) settles in the bottom of the tank. Most storage tanks used with gas wells have a liquid-removal outlet orifice near the bottom of the tank. The outlet orifice is typically about 16 inches above the tank bottom. As the quantity of oil builds up on top of the liquids in the tank, the quantity reaches the point where it should be removed and sold. Since the outlet orifice is at the bottom of the tank, the liquids below the oil have to be removed first.
When oil has accumulated to the point that it should be recovered, or if the water in the tank is too high, a water truck is called to the site. The water truck travels to the well pad, or wherever the storage tank is located, and removes the lower liquids (primarily water and the dirty oil and waste oil) to access the clean oil. After the water truck completes the removal of the lower liquids, the light valuable oil is at the orifice outlet and may be removed, if an oil truck is on site and available for removal of the oil.
To perform these operations on each storage tank when there is one or more storage tanks at a well site, compressor station or water plant, requires coordination of arrival of water trucks and oil trucks and, more importantly, involves considerable vehicular traffic. Many gas wells are located in remote areas that support wildlife and outdoors people. The vehicular traffic can be very disruptive and can significantly impact serenity and beauty of a pristine area.
Stratified liquids containing valuable light oil also are present at the compressor stations associated with gas wells. The gas from the well is piped to a compressor station for further processing before transportation to a distribution point for distribution to the end users. The separators used at the well sites remove most of the liquids from the gas. Nevertheless, some liquid remains, which contains light oil, water and contaminates, and is carried with the gas to the compressor station. At the compressor station, most of the remaining liquids are removed and placed in storage tanks. These liquids also stratify, with the light oil floating on the other liquids. Proper management of these liquids would also reduce vehicular traffic at the compressor station.
The water removed from storage tanks at gas wells and at compressor stations is presently transported by water truck to a water plant. This produced water is initially placed in produced water tanks for ease of off-loading the trucks, for storing the water and to control the flow through a down-stream heater/separator where some of the remaining oil is separated from the other liquids (primarily water).
Stratification takes place in the produced water tanks, and some of the clean oil is recovered after the lower level liquids are removed and sent to the heater/separator. The recovered clean oil is stored in an oil sale storage tank.
The clean oil at the output of the separator is also directed to an oil sale storage tank. The balance of the produced water at the output of the heater/separator is placed in settling tanks before transfer to an evaporation pond or to some water purification unit, such as a reverse osmosis unit. This water also contains some hydrocarbons that float on top of the water in the settling tanks and which is unwanted in the ponds and/or water purification units. Some of the clean oil is recovered and stored in an oil sale tank after the lower level liquids have been removed from a settling tank.
The amount of oil, dirty oil, and water in a produced water tank or a settling tank is not generally accurately calculated. A common method for determining the quantity requires an operator to ascend stairs to the top of a tank, open a hatch, and use one of the manual methods of roughly calculating the top surface level and bottom surface level of clean oil in a tank. A safer and more accurate and efficient way of measuring quantities is desired and a more efficient and accurate way of accessing and removing selected levels of liquids is desired.